TVo, Sheila, Dan Brown

Summary: Astigmatism in the OPO beam (that seems to be happening in HAM5 somewhere) is limiting the mode matching to the OMC. If the astigmatism can be fixed we could get 95% or better matching.

Along with the OMC mode scans taken the other night we also took beam profile measurement in HAM6 with the Nanoscan. On the OMC side of the table we took profile between HAM5 and OM1, OM1 and OM2, and on OMC REFL, on the OPO side we took them after ZM1 and the propagated off of the beam diverter onto SQZT6. Using the as built Finesse model for the HAM5 to OMC path I fit the input x and y plane beam parameters to the data. This is compared to the as built OMC mode propagated to the SRM AR surface (beam parameter values in the legend). 

The x-plane beam has an overlap of 83%, the y-plane 95%. Taking the ratio of the 2nd to 0th order peaks in the OMC scan will average the two planes, so we measure 89% matching. Which agrees pretty well with what we measured the other night. Although the OMC scan ratio should underestimate the mismatch in general due to astigmatism in the OMC, the non-zero 1st order peak from misalignment also couples a bit into the 2nd order modes, which causes an overestimate in the mismatch as it makes 02/20 larger. These two effects just happen to cancel each other out in these scans it seems.

I also compared our measurements to what I originally predicted we should have got. As can be seen the y prediction vs fit isn't that far off, but the x-plane astigmatism is causing it to be significantly different.

Lastly, I fit the beam profiles to a mode propagating away from ZM1. If there were no astigmatism (and the model parameters are correct) this beam propagated to the OMC would have ~98% matching.